Modern medical treatments often involve the use of intravenous fluids. The fluids may contain a wide variety of agents that affect the patient. The flow rate of medical fluid provided to the patient is critical in order to ensure that the patient is properly treated. Thus, it is important to provide a simple mechanism for adjusting the flow of a liquid in a tube. In U.S. Pat. No. 4,515,588, filed 16 May 1983, Amendolia suggests inserting an adjustable valve in the fluid path to control the flow rate. This prior art works with flexible plastic tubing. The tube is cut and an adjustable valve is inserted. The valve permits fine flow adjustment. Clearly, when such a valve is used, it is necessary to ensure that the sterilized plastic hose remains free of germs. Thus, a sterilized knife is used to cut the tubing. Clearly, the knife must be used immediately if it is to remain sterile. Although the tasks of cutting the tubing and inserting the flow regulator are relatively simple they take time and in a hospital environment, time is very costly.
Alternatively, in U.S. Pat. No. 3,215,394, filed 23 Apr. 1962, Sherman describes a simple method of controlling the flow of liquid in a flexible plastic tube by deforming the tube. The Sherman prior art relies on a cam that receives an external rotational input in order to interfere with the geometry of flexible tubing that the liquid flows through. This prior art does not require that the flexible plastic tubing be cut in order to control the flow of liquid therein. Unfortunately, the degree of control over the flow rate is poor. The device is simple however the variety of parts needed to build it and the assembly of those parts leads to a device that is fairly costly.
In U.S. Pat. No. 4,786,028, filed 24 Feb. 1987, Hammond describes a fluid flow adjustment mechanism much like the mechanism of the Sherman prior art. Hammond however inserts a block between the cam and the flexible tubing. This reduces the likelihood of wear on the tubing substantially, however the device does not provide the precise flow desired and demanded in a wide variety of medicinal applications.
This device is improved upon by D'Alessio et al. in U.S. Pat. No. 5,259,587 in which a fluid control mechanism is provided using a cylinder as a cam that pushes a surface of a second cylinder against a flexible tube. In this case, the deformation of the tube is controlled using a wheel with a ratchet mechanism. The ratchet prevents the cam from rotating out of position once it has been set. Although this mechanism offers a somewhat improved flow control mechanism over the related prior art, it also introduces additional components. Clearly, the components used will require some assembly and that cost will be apparent in the cost of the device.
Thus, a variety of prior art devices exist featuring a ram that moves perpendicular to a tube to deform the tube in order to adjust a flow of fluid within the tube. These devices are generally designed to work with special flexible tubing. The flexibility of flexible tubing is described by a durometer rating. A typical durometer rating of flexible tubing used in fluid flow adjustment devices for dental applications is 60 to 80 on a Shore A scale. This type of tubing is quite flexible and therefore relatively easy to deform. Harder tubing having Shore A durometer ratings of 85 and 90 are also quite common; however, these stiffer types of tubing are less well suited to use with this type of prior art device as they require substantially more effort to produce the desired deformation.
An alternative design causes three points bending in a flexible tube in order to control a flow of fluid therein. Thus, the flexible tubing is held by two supports and ram engages the tubing between the supports. The degree of engagement of the ram affects the fluid flow within the tube. Unfortunately, this method does not provide precise flow control.
Ideally, these devices would provide a more linear relationship between the position of an input control and the flow mechanism. For example, when the input is half way between the fully open position and the fully closed position ideally the flow provided would be half the flow associated with the fully open position. Thus, the position of the input would be directly proportional to the flow of liquid that the mechanism provides.
Ideally, a flow regulator should provide a consistent and directly proportional relationship between the displacement of an input and the flow rate. However, a consistent flow rate is not obtained with many of the prior art flow rate regulators because the pressure exerted on the wall of a flexible tubing is perpendicular to the fluid flow, which modifies the shape of the internal surface of the tubing without substantially modifying the flow of the fluid. A deformation from, for example, a circular shape to a somewhat elliptical shape does not change the flow substantially, even though a certain displacement is imposed on the wall of the flexible tubing to achieve such a deformation.
Clearly, it would be beneficial to provide a fluid flow adjustment device that provides highly precise flow control by deforming a flexible tube. Additionally, it would be beneficial if the device is simple and easily produced using very inexpensive components.